The number of products using secondary batteries keeps increasing, and secondary batteries are generally recognized as being essential for mobile devices such as mobile phones and notebook PCs. Among the secondary batteries, lithium ion secondary batteries are used widely since being small in size and having a large capacity, and are also employed in airplanes and automobiles. In recent years, research is actively conducted on lithium ion secondary batteries for the purpose of providing better lithium ion secondary batteries.
Regarding the safety aspect of lithium ion secondary batteries, ensuring safety when internal short circuiting occurs in a lithium ion secondary battery is important. A known method for confirming safety when internal short circuiting occurs in a battery is a nail penetration test for observing how a battery behaves when the battery is penetrated with a nail. In fact, Patent Literature 1 describes a lithium ion secondary battery that did not ignite when the nail penetration test was performed. The lithium ion secondary battery disclosed in Patent Literature 1 is obtained by segmenting an electrode into sheets having a specific shape.
In addition, Patent Literature 2 describes a lithium ion secondary battery that has: a positive electrode including a first positive electrode active material layer that contains LiNiO2 and a second positive electrode active material layer that contains Li2FeSiO4 or LiFePO4; and a negative electrode containing Cu6Sn6 as a negative electrode active material, wherein the amount of lithium in the second positive electrode active material layer is equal to or larger than an amount corresponding to an irreversible capacity of the negative electrode active material. However, the nail penetration test is not mentioned therein.